OBSERVATIONS OF THE MAGNETIC FORCE. 
269 
Needle F. C. 
Weights. 
1843. 
Low temperatures. 
High temparatures. 
Weights. 
1843. 
Low temperatures. 
High temperatures. 
t. 
V. 
f. 
v'. 
t. 
V. 
t'. 
v\ 
2‘0grs.-< 
r 
Feb. 2. 
3. 
3. 
6. 
6. 
4. 
24-7 
32-7 
31-0 
29-0 
28*1 
28-2 
26 32 
26 47 
26 38 
26 54 
26 45 
26 42 
83-0 
83-5 
67*8 
82-0 
64-6 
64*5 
26° 36 
26 49 
27 16 
26 58 
26 55 
26 42 
2’5 grs.< 
Feb. 2. 
3. 
3. 
6. 
6. 
4. 
28-0 
30- 7 
29-5 
29-0 
28-2 
31- 4 
34 06 
34 19 
33 54 
34 16 
34 15 
34 21 
88-0 
87*4 
70-1 
79*2 
64- 0 
65- 0 
34 33 
34 33 
34 24 
34 30 
34 30 
34 18 
Means. . 
28-9 
26 43 
72-9 | 26 52-7 
Means. . 
29*5 
34 12 
75-6 
34 28 
3'0grs.< 
Feb. 2. 
3. 
3. 
6. 
6. 
4. 
24-0 
29*1 
28-5 
29*0 
28-1 
33-7 
42 45 
42 31 
42 37 
42 41 
42 43 
42 42 
89*0 
87*2 
70-5 
73-0 
62-2 
65-0 
42 43 
42 38 
42 35 
42 40 
42 47 
42 56 . 
l - 5gr. - 
’ 
Feb. 6. 
6. 
28-5 
27-5 
19 39 
19 46 
82-1 
65-3 
20 12-0 
20 03-0 
Means. . 
28-7 
42 39*8 
72-8 
42 43-2 
Means. . 
28-0 
19 42-5 
78*7 
20 07*5 
Hence, giving- the observations with 1-5 gr. one-third the weight of those with eacli 
of the other weights, q — '00013 ; or, if we reject the observations with 3 grains, which 
are much more irregular than the others, ^ = -000175. The value which has been 
employed is *00016 for both needles F. A. and F. C. 
It will perhaps be most convenient to give a brief history of each needle during 
the period comprised by the observations discussed in this memoir ; beginning with 
F. A. of Mr. Fox’s apparatus, as having been the most extensively employed. The 
observations with this needle commenced at Woolwich, as a base station, on the 
7th of July 1842. In the transport in which Lieut. Lefroy made his passage to 
Canada in July and August, it was employed to give determinations of the Inclination 
and Force on all days when the weather permitted. The greater part of these deter- 
minations belong to a part of the globe which is not now under consideration, and 
will not therefore be discussed on the present occasion ; but a continuation of the 
series at some stations in Lower Canada, and at several in the United States which 
Lieut. Lefroy visited on his way to Toronto, for the purpose of connecting the 
base stations of other observers with his own future base station at Toronto, will be 
found in Table XII. The deflections obtained with this needle at Toronto on the 
26th of October 1842, with the same weights which had been used at Woolwich in 
July, gave for the ratio of the total force at Toronto 1*3395 to 1 at Woolwich ; or 
1*838 to 1*372 in terms of the usual arbitrary scale. Two subsequent determinations, 
one with the same apparatus in 1846, and the other derived from absolute measures 
of the horizontal Force at Toronto and Woolwich, gave each 1*835 to 1*372; these 
determinations have been discussed in the introductory remarks, pages 244 to 247, 
the result of the discussion being that 1-836 has been finally adopted. 
MDCCCXLVI. 2 N 
